SUMMARY Radiation-induced double-strand breaks (DSBs) are fundamental threats to genomic integrity that result in genomic instability if not properly repaired, which can in turn lead to cancer and cell death. Although we know a great deal about the pathways of DSB repair, we know very little about how DSB repair occurs in its natural context in the cell, that is, chromatin. Chromatin by its very nature is an impediment for proteins accessing the DNA, yet the repair machinery is somehow able to navigate through the chromatin and successfully repair DNA damage. Chromatin also plays a key role in transducing the cell's response to DNA damage via the DNA damage cell cycle checkpoint. Until recently, there has been a large gap in our understanding as to how the DSB repair and the DNA damage checkpoint are influenced by the chromatin environment in mammalian cells. Integral to this process is the progression of the DNA repair machinery along a genome that is packaged into chromatin; how this occurs and the influence on the epigenome, has been a long-standing mystery. We have recently shown that chromatin is completely disassembled and reassembled during non-homologous end joining of double-strand DNA breaks in human cells. Excitingly, our recent preliminary data strongly supports an active role for dynamic chromatin assembly onto single stranded DNA (ssDNA) in the midst of homologous recombinational repair of DSBs as an intrinsic step required for DSB repair. Histones occupying ssDNA has never been reported previously in vivo, let alone their playing an important biological role. The proposed studies will uncover the nature of the histone-DNA complexes on ssDNA, and will reveal the elusive mechanism whereby chromatin assembly promotes DSB repair in human cells. By elucidating the mechanism whereby ssDNA-histone complexes contribute to DSB repair, we hope to fill significant gaps in our current knowledge of the chromosomal repair process.